EVOLUTION 333 



structure which enables the larvae to feed is earlier and more 

 fundamental than those modifications for aquatic life which 

 differ within the limits of the order. From his study of the 

 air-tube system of dragon-fly larvae Tillyard (191 7) con- 

 cludes that the immature stages of primitive Odonata were 

 dwellers " in damp earth rather than in water." It is likely 

 that the forerunners of many other insect larvae, now 

 specialised for an aquatic life, passed ages ago through a 

 similar '' halfway house." 



The Ephemeroptera (mayflies) are of great interest from 

 the view-point of this discussion, because in them the 

 divergence between larva and imago reaches its greatest 

 extent among all the Exopterygota. The delicate short- 

 lived flies have vestigial jaws, so that they take no food after 

 acquiring their wings, of which the front pair are far larger 

 than the hinder. While their jaws and wings thus show 

 wide departure from the condition which must be regarded 

 as primitive among insects, they retain the far-off ancestral 

 character of paired openings to the genital ducts, which in 

 all other winged insects open into a single terminal passage. 

 They also present the archaic character of long, jointed 

 cercopods. Their aquatic larvae, as previously pointed out 

 (p. 185), are essentially thysanuroid insects modified for 

 life in the water, as they agree with the Apterygota in 

 possessing mandibles of the crustacean type ; their long 

 feelers and cercopods recall those of typical bristle-tails, 

 and their series of paired abdominal tracheal gills are the 

 appendages of that region modified in correspondence with 

 hfe under water. They are therefore the one order of 

 winged insects which clearly show a special relationship 

 to the Apterygota, and their larvae, if divested of their special 

 aquatic adaptations, may be reckoned as furnishing some 

 indication of the wingless stock whence the earliest winged 

 insects were derived. It will be remembered that at the 

 close of their transformations, mayflies afford an example 

 through the sub-imago, unknown in any other order, of a 

 moult after acquiring the power of flight. This was 

 probably a common condition among the primitive winged 



